Rectification of alternating currents



y ,1 K. B. MCEACHRON 1,857,160

RECTIFICATION 0F ALTERNATING CURRENTS Filed Feb. 29, 1928 2 Sheets-Sheet 1 Inventor:

Karl B. McEQChmn;

b MZZQ/M His Attorneg.

y 1932- K. B. MCEACHRON 1,857,160

RECTIFICATION OF ALTERNATING CURREN'I'S Filed Feb. 29, 1928 2 Sheets-Sheet 2 Pi .8. Fig.9. 25 23 5 i l film 2 I (a 25 6 Inventor: Kaml B. McEachvon,

by Wm Hi5 Attorney Patented May 10, 1932 UNITED STATES PATENT OFFICE KARL B. MCEACHRON, F PITTSFIELD, MASSACHUSETTS, ASSIGNOR TO GENERAL ELEC- TRIC COMPANY, A CORPORATION OF NEW YORK RECTIFICATION O1? ALTERNATING CUBRENTS Application filed February 29, 1928. Serial No. 258,112.

My invention relates to the rectification of alternating current, and has for its principal object the provision of an improved apparatus and method of operation whereby an al- 5 ternating voltage is distorted and modified to produce a current having a comparatively large unidirectional component.

This application is a continuation in part of my copending application Serial No.

198,512, filed June 13, 1927, and is assigned to the same assignee as the copending application. The copending application relates 'more particularly to a resistance material which possesses the characteristic that a comparatively large change in the current transmitted through it is produced by a comparatively small change in the voltage to which the material is subjected. Further, the resistance material possesses the characteristic that it has no time lag, or a time lag that is negligible. This material, as pointed out in the copending application, may consist of silicon carbide or a mixture of silicon carbide with materials such as tungsten, molybdenum, galena, clay, glass, or the like.

A general equation derived from Ohms Law for this resistance material may be written as follows: RI*=C. In this equation, the exponent a represents the slope of the resistance-ampere curve with reference to the log-log-coordinatesand c represents the resistance when the current is equal to one ampere.

The resistance of the material varies in accordance with some power of the voltage to which it is subjected. If it is assumed to have an exponent of 0.8 for example, the current increases as the fifth power of the voltage applied to the material. This means that, if the voltage applied to a circuit has unequal positive and negative crests, the in sertion of the resistance material in the circuit will increase the area of the lobe having the higher crest value and will decrease the area of the lobe having the lower crest value. In accordance with my invention, this char acteristic of the material is utilized to produce a current which is substantially unidirectional in character.

My invention will be better understood from the following description when considered in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

Referring to the drawings, Figs. 1 to 4: and 8 to 11 illustrate different apparatus wherein my invention has been embodied; and Figs. 5 to 7 and 12 relate to the operation of these apparatus.

Fig. 1 shows a rectifying apparatus comprising a core 1 which is provided with an exciting winding 2 arranged to be connected to a suitable direct current source through a resistor 8 and with a winding 4 arranged to be connected to an alternating current source through a block of the resistance material previously described and a load 6 to which direct current is to be supplied.

If the voltage of the alternating current source is applied directly to the winding 4;, a resulting current wave similar to the curve 7 of Fig. 5 is produced. This wave has positive and negative lobes of equal area but is distorted by the direct current flux of the winding 2 so that its positive crest value is much larger than its negative crest value.

If the block of resistance material is connected in circuit with the winding 4:, the current wave is modified and assumes the form of the current wave 8 shown by Fig. 6. It will be observed that the positive lobe of this wave has a much larger area than the negative lobe. This shows a predominance of current in one direction. The curves 7 and 8 are reproduced from actual oscillograms taken under the conditions set forth above.

The use of an apparatus such as that illustrated by Fig. 1 has the disadvantage that an alternating voltage is induced in the direct current winding 2 and a rather high voltage must be applied to this winding in order to produce the required direct current flux in the core. The series resistor 3 absorbs a large part of this induced voltage. This resistor may obviously be replaced by a reactor which will absorb the induced voltage without producing the resistance loss which occurs in the resistor. It will be apparent that a tuned circuit or wave trap may also be utilized in the direct current circuit for absorbing the induced voltage.

Fig. 3 shows an apparatus wherein the effect of the induced voltage is largely eliminated. This apparatus comprises two cores 9 and 10 provided with two alternating current windings 11 and 12 so arranged that the voltages induced by the two alternating windings tend to cancel one another. Such a cancellation, however, will not be complete because the voltage wave contains a large second harmonic which can not be cancelled out by such a method. It is therefore still necessary to provide an impedance device of some sort in series with the excitation winding 2 or to provide some other means of absorbing the alternating potential 111-- duced in the excitation winding.

Instead of depending on the direct current excitation winding for the distortion of the current wave, the same result may be produced by the arrangement of permanent magnets shown by Fig. 2. In this arrangement, the permanent magnets 13 and 14 are so arranged as to produce a certain constant magnetomotive force in a core 15 upon which the alternating current coil 4 is wound. This method of rectification does not require the use of a direct current exciting winding but depends altogether on the magnetic saturation produced in the core 15 by the addition of the magnetomotive force of the permanent magnets 13 and 14 and the alternating current winding.

The principle involved in the production of the distorted current wave is well understood by those skilled in the art. As indicated by Fig. 7, the axis of the core magnetization is at mm when no direct current excita tion is utilized and is at his when direct current excitation is utilized. With the axis at Zola, an increase in the voltage of the coil 4 in one direction will produce a comparatively large change in current while an increase of this voltage in the opposite direction will cause a comparatively small change in current due to the larger counter electromotive force which exists when the direct and alternating fluxes are in opposite directions.

Any circuit comprising a poor rectifier having high leakage or large reverse current will be improved by the inclusion of a block of the resistance material connected in series with it as indicated in Fig. 4. In this figure, a poor or leaky rectifier 16 of any suitable type is arranged to be connected to an alternating current source through a block of resistance material 5 and the load 6. When large currents tend to flow, the resistance of the block 5 is low; and when small currents tend to flow the resistance of the block 5 is high. The effect of the resistance is thus to decrease the reverse or leakage current and to offer com paratively little opposition to the flow of current in the normal direction.

The arrangement illustrated by Fig. 11 comprises a composite core including a part 17 made of silicon steel or the like, a part 18 made of a permanent or direct current excited magnet, and a part 19 made of a material such as nicoloi or permalloi which has a high permeability at low flux densities. In Fig. 12, the magnetic characteristics of the core member 19 are illustrated by the curve 21 and the magnetic characteristics of the core member 17 are illustrated by the curve 22.

In the operation of the arrangement illustrated by Fig. 11, the core member 17 provides a return path for the alternating flux. Normally the direct current flux would enter the core member 17 and saturate it to the same extent as it saturates the core member 19 but this is prevented by making the core member 19 of a material which has high permeability at low flux densities and making the member 17 of a material such that its flux density is less than that of the core member 19 for the same magnetizing force. Furthermore, the core member 17 may be made of larger cross section than the core member 19 so that, when the flux in the core member 19 changes from its biased positive saturation to negative sat uration, the flux in the core member 17 does not quite saturate this member. Under these conditions, the alternating flux will follow the core member 17 but the controlling reluctance in the circuit will be that of the core member 19 with the winding 4 producing the desired saturation eflt'ect.

Figs. 8 to 10 illustrate various Ways in which the resistance material 5 may be utilized in combination with a reactor 23 for the purpose of supplying rectified current to the load 6.

Fig. 8 shows an arrangement in which the reactance and resistance material are connected in series and the load or battery 6 is shunted across the reactor 23 in series with a member 24 of the same resistance material as the member 5. Since the voltage across the reactance 23 is higher on one side of the cycle than on the other, rectification is produced by this combination.

As indicated by Fig. 9, the load 6 and a resistance 25 may be connected in shunt to the resistance material 5 which is connected in series with the reactor 23.

With this arrangement, the load 6 receives current during the half of the cycle in which the greater voltage appears across the reactance. For a short time in the following half cycle, current will flow through the load but this time will be so much shorter than the corresponding time in the preceding half cycle that there is a large direct current compo- 1 nent. Rectification is thus achieved by the comparatively long time of current flow during alternate half cycles. It should be noted that in the reversed cycle a comparatively large current will flow through the resistance material, this being essential to proper operation of the apparatus.

As indicated by Fig. 10, rectification of both half cycles of the wave may be effected by the provision of condensers 26 and 27.

The embodiment of the invention illustrated and described herein has been selected for the purpose of clearly setting forth the principles involved. It will be apparent, however, that the invention is susceptible of being modified to meet the different conditions encountered in its use and I therefore aim to cover by the appended claims all modifications within the true spirit and scope of my invention.

lVhatI claim as new and desire to secure by Letters Patent of the United States is:

1. In a rectifier system, a magnetic core, means comprising an exciting winding for producing a substantially constant magnet-0- motive force in said core, a circuit including a source of alternating current and an alternating current winding wound on said core,means for absorbing the voltage induced in said exciting winding by the current of said alternating current winding, and a member com prising silicon carbide connected in series with said alternating current winding, whereby said alternating current is distorted and modified to produce a current having a large unidirectional component over a range of frequencies including relatively high frequencies and the modifying of said alternating current due to said resistance member is as effective at said high frequencies as at low frequency.

2. In a rectifier system, a magnetic core, means comprising an exciting winding for producing a substantially constant magnetomotive force in said core, a circuit including a source of alternating current and an alter nating current winding wound on said core, means for absorbing the voltage induced in said exciting winding by the current of said alternating current winding, and a member composed of resistance material connected in series with said alternating current winding, said member having resistivities which at one temperature differ when it is subjected to voltages from said source having different crest values and characterized by a time lag which is substantially zero, whereby said alternating current is distorted and modified to produce a current having a large unidirectional component over a range of frequencies including relatively high frequencies and the modifying of said alternating current due to said resistance member is as eifective at said high frequencies as at low frequency.

8. In a rectifier system, a magnetic core, a circuit including a source of alternating current and a winding on said core, means for producing a substantially constant magnetomotive force in said core, means for impeding or preventing the entrance of other fluxes to the portion where said constant magnetomotive force originates, a member composed of resistance material having a resistivity which at one temperature is comparatively high when it is subjected to one voltage and is comparatively low when it is subjected to another voltage and characterizcd by a time lag which is substantially zero, and means for connecting said winding in series with said member, whereby said alternating current is distorted and modified to produce a current having a large unidirectional component over a range of frequencies including relatively high frequencies and the modifyin of said alternating current due to said resistance member is as effective at said high frequencies as at low frequency.

4. In a rectifier system, a core having a winding, a circuit including an alternating current source and said winding, a means for producing unidirectional component of flux through the core, means for preventing the varying fluxes from entering the portion of the core where the unidirectional fluxes originate, a. member composed of resistance material having a resistivity which at one temperature is comparatively high when it is subjected to one voltage and is comparatively low when it is subjected to another voltage and characterized by a time lag which is substantially zero, and means for connecting said winding in series with said member, whereby said alternating current is distorted and modified to produce a current having a large unidirectional component over a range of frequencies including relatively high frequencies and the modifying of said alternating current due to said resistance member is as effective at said high frequencies as at low frequency.

In witness whereof, I have hereunto set my hand this Q'Zt day of February 1928.

KARL B. MGEACHRON. 

